The rapid development of the telecommunications industry have made wireless handheld devices like cell phones, pagers, two-way messaging devices, etc. massively popular, creating a need for new electronic components and circuits in both mobile and base station systems as competition drives the introduction of expanded capabilities.
Radio frequency power amplifiers contribute to the power consumption in the communications systems. To extend battery life in mobile units, and to reduce operating costs of base stations, there is a need to develop new amplifiers to replace the traditionally inefficient, power wasting, elderly designs currently in use.
Many contemporary base station amplifiers employ complex techniques to realize amplifiers with a high degree of linearity over a broad frequency range. Unfortunately those solutions have a low efficiency.
Handset power amplifiers also suffer from efficiency problems, often more critical than those for base stations as the power supply for mobile user equipment is strictly limited. Today's smaller, faster and more effective portable electronics demand high power with only little losses.
For many decades, linear power amplifiers of the Class A and/or Class AB type have been employed as radio frequency power amplifiers for cellular base station systems. These amplifiers are based on the operation of a transistor in its linear mode. As such, they are however limited in their ability to efficiently amplify radio frequency signals.
The Doherty amplifier is another common method to increase the efficiency in the context of power amplification. However, the Doherty method is often demanding to implement. In practical cases a Doherty amplifier seldom reaches an efficiency level above 50%.
Switching mode amplifiers have been used for quite some time in various electronic systems including audio power amplifiers and switching power supply circuits. In switching mode amplifiers, the transistor is operated as a switch. Switching mode amplifiers suffer however from the existence of some fundamental limits, such as large parasitic capacitance, which may prevent many transistors from working well at high power and frequency.
The Class-D amplifier architecture has often been used in low frequency and audio frequency applications. The traditional Class D amplifier, or Voltage Mode Class D amplifier, is defined as a switching circuit that results in the generation of a half-sinusoidal current waveform and a square voltage waveform. Voltage Mode Class D amplifiers are however afflicted with a number of problems that make them awkward to realize. Firstly, the availability of suitable devices for the upper switch is limited. Secondly, device parasitics such as drain-source capacitance and lead inductance result in losses in each cycle.
The Current Mode Class D amplifiers that has emerged, has proven to be feasible to implement also at high frequencies and have output levels with good bandwidth and peak drain efficiency. However, it is very important to optimize the harmonic terminations in the output filter to get the square current wave form and the half wave sinusoidal voltage waveform to minimize overlaps. Nearly ideal waveforms are critical to maximizing efficiency, as an overlap of the current and voltage waveforms correspond to power dissipated in the switch and cause rapid efficiency degradation.
Thus there is a problem to develop a Current Mode Class D amplifier with harmonic terminations in the output filter to get the square current wave form and the half wave sinusoidal voltage waveform to minimize overlaps, in order to reach a high degree of efficiency.
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. Amplitude modulation works by varying the strength of the transmitted signal in relation to the information being sent. For example, changes in the signal strength can be used to reflect the sounds to be reproduced by a speaker. Unfortunately, Current Mode Class D amplifiers loose efficiency when working in back off i.e. working below full effect, why they at present are unsuitable to be used for amplitude modulation of a radio signal. Thus there is a problem with efficiency in back off.
Another problem with Current Mode Class D amplifiers is that they occasionally may suffer from the occurrence of large voltage spikes when symmetrically pulse width modulated. This prevents practical usage since the switching units can break because of the large voltage spikes.